The present invention relates in general to misfire detection in internal combustion engines, and more specifically to a system and method that corrects for irregularities in measured crankshaft velocities resulting from torsional flexing of the crankshaft and errors in the placement of position markers in a crankshaft position sensor.
In crankshaft based misfire detection methods, the failure of one or more cylinders to produce power during its respective power stroke is detected by sensing very small changes in the velocity (and thus acceleration) of the engine crankshaft. Since the velocity during each velocity measuring interval of engine rotation is determined according to the rotational arc .DELTA..theta. covered by the interval divided by the time .DELTA.T required to pass through the arc, the measured values for both .DELTA..theta. and .DELTA.T must be measured sufficiently accurately to provide the sensitivity required to detect such small velocity changes.
Crankshaft based misfire detection methods are discussed in U.S. Pat. Nos. 5,044,194, 5,056,360, and 5,109,695, all hereby incorporated by reference. Such methods perform well under certain engine operating conditions. However, at certain combinations of engine speed and engine load, the accuracy of crankshaft based misfire detection methods typically decreases. For example, with an engine operating at high speed and low load, irregularities in the measured crankshaft velocity interfere with reliable detection of misfires or proper firings of individual cylinders.
A significant source of such irregularity is position errors in determining the rotational arcs during each measuring interval. Engine rotational position is monitored using a rotor having vanes, teeth, or slots disposed thereon for interacting with magnetic or optical sensors at predetermined points in the rotation of the engine crankshaft. One source of position error results from the rotor wheel profile. During manufacture of a wheel, errors occur between the desired and actual positions for position markers on the wheel. Any deviation of the actual angle .DELTA..theta. from the assumed value results in velocity and acceleration errors. This type of position error is discussed in U.S. Pat. No. 5,117,681, which is incorporated herein by reference. Another source of irregularity is the torsional flexing of the crankshaft during engine operation. The crankshaft is an extended shaft constructed of metal which is not completely rigid and thus flexes as it is driven by the engine cylinder firings. A crankshaft typically is connected to a flywheel of large mass near the back of an engine and extends through the front of the engine for access by auxiliary components. The torsional flexing along the crankshaft creates oscillations in the sensed crankshaft rotation intervals. These oscillations again lead to irregularities in the resulting velocities and accelerations, possibly causing inaccuracies in the misfire detection.
Corrections are known for each source of velocity irregularity. U.S. Pat. No. 5,117,681 discloses a method for measuring wheel profile errors in an individual engine and storing correction factors used during misfire detection to remove the position errors. U.S. Pat. No. 5,237,862, incorporated herein by reference, discloses an adaptive method for correcting for wheel profile errors that derives the correction factors during actual engine operation. The misfire detector attempts to identify periods when no misfire occurs and then smooth out the variations in cylinder accelerations to remove the fluctuations caused by the wheel profile errors.
Reduction of the effects of crankshaft torsional disturbances using torsional correction factors derived at various engine speeds and loads has been demonstrated in U.S. Pat. No. 5,377,537. Thus, it is known that the torsional oscillation includes a periodic component that is substantially constant at each separate constant speed and load point of the engine. By measuring the torsional oscillations in a test engine, correction factors are determined and stored in a look-up table to be used in production engines for misfire detection during normal vehicle use. Although a test engine characterizes the expected torsional oscillations for a particular engine design reasonably well, engines of that same design may experience changes in torsional flexing as the engines wear with use. Furthermore, in some circumstances it may be desirable to eliminate the need for measuring test engines and storing look-up tables associated with each different engine design and the resulting manufacturing complexities.
Adaptive correction methods in misfire detectors must overcome several difficulties. The basis of adaptation is that the correction factors can be derived during a time when misfire is not present. Thus any adaptive learning of correction factors must be disabled under conditions of misfire. However, detection of a misfire depends upon the capability of the correction which itself depends upon the ability to detect a misfire, leading to a circular problem that adaptation cannot be achieved until adaptation is achieved. In addition, if updating of the learned correction factor is cut off during conditions of misfire and if engine conditions are changing during that same period, the latest available correction factors may become inaccurate. Furthermore, it is possible that the adaptive correction may adapt to a gradual onset of power loss in a cylinder and thus become incapable of detecting an actual power loss that has reached the required threshold.